Allow function types to be registered without workarounds

Allow function types to be registered without workarounds

Commit 4b8ceb41aed352f10d36db5284453f425dbc5f3f added the requirement
that pointed-to types need to be registered when registering pointer
types. Unfortunately, the implementation also affects function pointer
types.

This change whitelists 0, 1, 2 and 3 argument functions as not deriving
from QObject, forgoing the need to workaround details of the type
registration implementation when registering those function pointer
types.

Change-Id: I4d855e9d70a8179a6e31b84623ad5bf063e0d6d8
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>